Pedal Ratio for Manual Brakes

[BlackdogNY]

Trying to choose a pedal assembly, narrowed it down to (2) Tilton pieces...(1) floor mount, other is overhung. Like the floor mount better, but pedal ratio only go as high as 5.75:1...hanging go to 6.2:1.
From what I have researched, pedal feel and travel length can be 'adjusted' thru MC size...smaller MC>pedal travel.
Question is, with the lower mechanical advantage of the 5.75:1 pedals, will a smaller MC make up the difference....and is there a disadvantage to having a 'smaller' MC.
Car will be light<3000lbs, 295f/335r BFG's, Wilwood 13" 6 piston f/12" 4 piston r
Not sure on pad material, did not have a option to choose.
Car will see limited street use and occasional trackdays...hopefully.

Thanks for the help.
B&G

[malibudave]

What is the recommended master cylinder bore size from Wilwood?

For example:
If Wilwood recommends a 3/4" (0.75") bore master cylinder for the front, and your leg effort is around 100 ftlb, then the results of psi output will be:
6.2 pedal ratio = 1404 PSI
5.75 pedal ratio = 1302 PSI

I used this formula to calculate the PSI
ftlb input from leg / master cylinder piston area X pedal ratio

OR

100 ftlb input from leg / ((.75 / 2) X (.75 / 2)) X 3.14) X 6.2 = 1404 PSI with a 3/4" bore master cylinder and a 6.2 pedal ratio.
100 ftlb input from leg / ((.75 / 2) X (.75 / 2)) X 3.14) X 5.75 = 1303 PSI with a 3/4" bore master cylinder and a 5.75 pedal ratio.
108 ftlb input from leg / ((.75 / 2) X (.75 / 2)) X 3.14) X 5.75 = 1406 PSI with a 3/4" bore master cylinder and a 5.75 pedal ratio.

Hopefully my math is correct.

INCREASING master cylinder bore size will REDUCE the PSI output.
At those level of PSI, in my opinion, you will not know much difference in performance.

The 6.2 pedal ratio will give LESS THAN 1.0" of master cylinder piston travel. This equals LESS volume of fluid being moved. You will have a longer pedal stroke to move the same volume of fluid.

The 5.75 pedal ratio will give MORE THAN 1.0" of master cylinder piston travel. This equal MORE volume of fluid being moved. You will have a shorter pedal stroke to move the same volume of fluid.

[BlackdogNY]

Have not asked Wilwoods recommendation yet...not even seen completed suspension with brakes. Only going by what was offered when suspension systems were ordered.
Every time I research this it gets more confusing with so many different variables that effect pedal feel and travel distance. At this point some of the variables are easier and less costly to change. I needs pedals/mc's...want extra room/weight savings of manual brakes...hoping a open canvas of a braking system is enough to design something with max. performance and reasonable comfort.
Not going to buy new calipers and rotors until it is proven that these will not work to their potential.
Car heads to chassis shop soon will get their opinion as well.
Thanks for the help.

B&G

[Apogee]

What is the recommended master cylinder bore size from Wilwood?

For example:
If Wilwood recommends a 3/4" (0.75") bore master cylinder for the front, and your leg effort is around 100 ftlb, then the results of psi output will be:
6.2 pedal ratio = 1404 PSI
5.75 pedal ratio = 1302 PSI

I used this formula to calculate the PSI
ftlb input from leg / master cylinder piston area X pedal ratio

OR

100 ftlb input from leg / ((.75 / 2) X (.75 / 2)) X 3.14) X 6.2 = 1404 PSI with a 3/4" bore master cylinder and a 6.2 pedal ratio.
100 ftlb input from leg / ((.75 / 2) X (.75 / 2)) X 3.14) X 5.75 = 1303 PSI with a 3/4" bore master cylinder and a 5.75 pedal ratio.
108 ftlb input from leg / ((.75 / 2) X (.75 / 2)) X 3.14) X 5.75 = 1406 PSI with a 3/4" bore master cylinder and a 5.75 pedal ratio.

Hopefully my math is correct...

Close...but not quite. Since you're dealing with a dual master cylinder setup and balance beam, you would need to divide your input force between the two master cylinders, so if 100 pounds were applied and the balance beam centered, then you would have a 50 pound force into each MC.

Ultimately, you can just compare the pedal ratios assuming all of the other variables stay the same to get a feel for the difference it will have. A 6.2:1 pedal ratio is almost 9% lower than a 5.7:1 pedal ratio, so it will require 9% less effort and 9% more pedal travel to achieve the same brake torque as a 5.7:1 ratio setup.

Tobin
KORE3

[malibudave]

Thanks Tobin. I didn't think to divide it in half for dual master cylinders.


A 6.2:1 pedal ratio is almost 9% lower than a 5.7:1 pedal ratio, so it will require 9% more effort and 9% less pedal travel to achieve the same brake torque as a 5.7:1 ratio setup.

6.2:1 pedal ratio is higher than a 5.75:1, correct? It will take less effort than and more pedal travel, correct? Maybe I am misunderstanding what the definition of brake torque is. Thanks for your insight.

[BlackdogNY]

"6.2:1 pedal ratio is higher than a 5.75:1, correct? It will take less effort than and more pedal travel, correct? Maybe I am misunderstanding what the definition of brake torque is. Thanks for your insight."

See why this can be so confusing to the average person. Add in MC size, pad friction material, caliper piston surface area....it gets even worse.

Thanks for the replies, just trying to learn a little.
B&G

[a67]

See why this can be so confusing to the average person. Add in MC size, pad friction material, caliper piston surface area....it gets even worse.

Thanks for the replies, just trying to learn a little.
B&G

Up in the brake sizing tutorial sticky, page 7, post 140, I attached spread sheet (SS) to do the brake calculations for you. With it you can plug in various parameters and see the effects it has.

Bob.

[BlackdogNY]

It would not load on my computer...will try again as soon as I can get the specifics for the brakes themselves.

B&G

[RobNoLimit]

Just a tip. I know that using the standard of 100 pounds 'leg' input is normal to compare brake systems. It is my belief that this practice was started by brake companies to show you big psi numbers. I could be wrong. Fed. DOT used to mandate passenger car testing at 60 lbs on the pedal (this was in the '70's, I have no idea what it is now). But, if you want to understand the truth about whats happening in the drivers seat, try this out. Set a bathroom scale up against the wall, lean it up to the wall at 10 or 15 degrees. Set a chair about 24" back (a lower chair is better). Sit in the chair and put your foot on the scale, heal on the ground, ball of your foot in the middle of the scale. Now, push on the scale with one foot. OK, you don't have a steering to hold on to, and your not strapped in, but you are sitting higher, so gravity is on your side. If you can get up to 100 lbs, try to modulate up/down 10 to 15 lbs either side of 100. Now try it at 60 lbs. It's true that 100 lbs gives you a way to compare, but this is not really the pressures that the system will be operating at under most street and track conditions. I'm not trying to bash anyone, because there is a lot of great help here and these guys know their stuff. Just wanted to get you to think about the actual conditions on the street. - BTW, 5.75 will be fine.

[BlackdogNY]

I do appreciate all of the input...still confused...getting better.
Next 'issue' is which pedal design to install. I do like the floor mount style, but curious what some other people have actually used.
Have not driven floor pedals since my 'Bug' daily drive a long time ago.
Also, which style is easier to mount and work on for maintenance?? Worried hanging will have to fit around steering column where floor can be located where ever.
How about mounts that are adjustable for different drivers? Is it better to have seat move? pedals? or both??
I did see a67's install, mine is a bit different, it is a tube chassis and want reservoirs out of engine compartment.
Lots of articles about installing pedal boxes, very few have actual photos/instructions showing what needs to be done. Saying 'rigidly mounting' is not a real help.
Trying to figure a way of mounting to a 'plate' that then could be 'rigidly mounted' to chassis vertically and horizontally...with possible adjustment.
Factory Five has a nice video on installing Wilwoods in a Mk4...https://youtu.be/CNjDsEbqJVk.
Anybody make a 'universal' mount for Tilton similar to the FF design?

B&G

[malibudave]

Just a tip. I know that using the standard of 100 pounds 'leg' input is normal to compare brake systems. It is my belief that this practice was started by brake companies to show you big psi numbers. I could be wrong. Fed. DOT used to mandate passenger car testing at 60 lbs on the pedal (this was in the '70's, I have no idea what it is now). It's true that 100 lbs gives you a way to compare, but this is not really the pressures that the system will be operating at under most street and track conditions. I'm not trying to bash anyone, because there is a lot of great help here and these guys know their stuff. Just wanted to get you to think about the actual conditions on the street. - BTW, 5.75 will be fine.

I would have to agree with you. I use 100 as a nice round, camparable, standard number, but I would think that 60 ftlb would be more realistic number.

[Apogee]

Sorry for the confusion I caused above, I had my more/less backwards with respect to pedal effort and travel, but it's corrected now.

As for effort, I've read somewhere (NTSB spec maybe) that all vehicles must be able to achieve lockup at less than 115# pedal force, but that is a lot of force. While most adults can usually apply up to 100 pounds on the brake pedal without too much difficulty, especially in a panic stop type scenario, doing it repeatedly certainly can be fatiguing. That said, while it's a nice round number that makes the math nice and easy for comparison purposes, it isn't what I would aim for when setting up a brake system. Most manual systems I've worked with will aim to achieve lockup near the 70-90 pound mark, as dictated by the pedal ratio, MC bore, caliper piston area, rotor diameter and pad CoF relative to the traction available. Power brakes will clearly achieve lockup at lower pedal efforts proportional to the gain factor.

Tobin
KORE3

[a67]

It would not load on my computer...will try again as soon as I can get the specifics for the brakes themselves.

B&G

That is strange, I use an older version of Excel so there isn't a need for the latest version. It may be an anti-virus program on your PC that is preventing it from running. I have run into that with files that have been downloaded from the 'net.

Bob.

[a67]

Regarding pedal pressure, the DOT is also assuring that both young and older folks can safely operate the brakes. Think of the little old lady driving to church on Sunday.

And unless you are on a track one won't be applying 100#'s of force every time the brakes are used.

Bob.

[BlackdogNY]

Ended up purchasing the Tilton Floor Mount Pedals, with mc's towards firewall. Choose them because they seemed to leave the most room for steering shaft and will also allow for a adjustable mount. Tilton recommended bolting the pedals to a plate and them, if you want to have adjustment, make the whole plate adjustable. Aluminum plate 1/4" and up will work...had some 5/16" 6061...so choice was easy. Will have to deal with pedal travel/feel issues when project gets there.
Only issue I have is getting use to the floor mount design. Feet are not large enough to use my heal as a pivot, especially on throttle...so most likely will make a platform in front of pedals.
They look real cool if that makes any difference....

B&G